I use Adobe Audition and changing levels is very easy. In Audition they will show you maximum, minimum, average and total RMS power under amplitude statistics. I target average RMS power around -15 to -17 dB FS and will occasionally alter the channel balance.

By ear is best, but for many types of music/recordings (not all) it is far easier if you listen to the difference channel (left channel minus right channel) while adjusting the relative levels of the two channels. A perfect balance will cancel-out (null-out, remove) anything that is panned to the centre. With typical vinyl, it won't cancel out perfectly due to distortion, but it's audibly obvious when you hit the minimum point, which is the point at which the channels are balanced as well as they can be.

I don't know Sound Forge 10, but I'm sure it has this function.

Obviously you don't want to save left minus right, but want to adjust the channels by exactly the same amount that made left minus right cancel-out the central instruments/vocals.

Left minus right is also sometimes called vocal cut, though there are better ways of doing a vocal cut, so if SF offers those, don't use them for this trick as you want a perfectly simple left minus right.

GoldWave ($50 USD) has a tool called Max/Match. It makes the left & right averages equal, and at the same time normalizes (maximizes) for 0dB peaks on the channel with the highest peaks (after balancing).

(Since the difference between peak & average is different for each channel, usually only one channel will have 0dB peaks after balancing & normalizing.)

I only use that when I feel there is a problem, because you could have a situation where one channel is supposed to be louder/quieter for part of the song and matching the overall-average levels could throw things off.

If you want to do this manually -1. Measure the left & right averages.2. Match the channels by decreasing the channel with the highest average.3. Measure the left & right peaks.4. Increase both channels by the same amount as required (if required) to normalize the channel with the highest peak.

(If you want to go through the math, that can all be done by taking the 4 measurements first and making one volume adjstment to each channel.)